A general projection system may be classified as front projection based or rear projection based depending on the positions of the viewer and projector relative to the screen. In a front projection system, the viewer and projector are on the same side of the screen, with the image from the projector reflecting off the screen to the viewer. In a rear projection system, the projector and viewer are on opposite sides of the screen, with the image from the projector being “transmitted through” the screen to the viewer.
FIG. 1 shows a prior art rear projection system 21 and a prior art front projection system 23. As shown, the image projectors 25 can be placed at different positions in respect of the screen 20. The throw ratio is given by the projection distance, d, divided by the screen diagonal length D, or:
                              Throw          ⁢                                          ⁢          Ratio                =                  d          D                                    (        1        )            
A common design goal in building any projection system is to minimize the throw ratio, without sacrificing image quality. As discussed, the throw ratio is defined as the ratio of the distance from the screen of the farthest optical element (often the projector) to the size of the projected image as given by the image/screen diagonal. Minimizing the throw ratio is especially important for rear projection systems in which the projector and screen are physically combined into a single functional unit, such as rear projection televisions. In such units minimizing the throw ratio implies a smaller cabinet depth, which houses the screen and projector. Minimizing the throw ratio of front projectors also provides other important advantages, such as the ability to locate a projector close to screen surface allows ease of placement and avoids interference of the light path by presenters or audience.
To decrease the throw ratio, prior art methods have combined planar mirrors with low distortion and wide field of view (FOV) lenses to fold the optical path, which serves to decrease the projection distance, hence decreasing the throw ratio. By fine-tuning the optical geometry (lens type, focal distances, mirror angles), it can be assured that image distortions are minimized. This has the disadvantages of requiring optical elements that are difficult to design and expensive to manufacture and restricting the sizes/placement of the optical elements. The optical and geometric constraints manifest as pincushion or barrel distortion and keystone distortions. The design of prior art systems have largely been constrained by the requirement of minimizing distortions.
More recently curved mirrors have been used, along with a computational circuit that serves to eliminate distortions such as in U.S. Pat. No. 6,233,024 to Hiller et al. However, the invention disclosed in U.S. Pat. No. 6,233,024 is restricted to optimizing only rear projection systems with constraints (due to minimizing distortion), though reduced, still lingering. Further, the mirror orientation angles are constrained to a certain range and confined to a single projector system. Finally, U.S. Pat. No. 6,233,024 is based on a projection mechanism that generates an image on a screen using scanning laser light bundles, where the computational circuit controls the deflection and intensity of the light bundles, which is a cumbersome and inflexible arrangement that provides limited fine-tuning of the data.
Current CRT based projection systems generate images in a raster scan format controlled electronically by horizontal and vertical deflection circuits. These deflection circuits incorporate compensation circuits that generate non-linear deflection control signals to compensate for the non-linearity between deflection angle to display surface scanned area. This distortion results in the display of a pincushion image. Typically, the compensation circuits can be adjusted to also compensate for distortion of lenses or other distortion anomalies in the projection system. For newer generations of projection systems employing fixed matrix displays, specifically microdisplays, such a compensation method cannot be accommodated. In addition, a projection system utilizing microdisplay(s) requires an optical magnification of approximately 100 times to illuminate an image of 60–70 inches. This requires high tolerance in alignment and calibration of the projection optics in production and adjustments due to misalignment from shipping and aging.
Finally, inherent in any wide angle and off-axis projection system, is the large variation in optical path within the optical envelope. This and other optical component anomalies (e.g. light source, display devices, lenses, etc.) can result in the projected image having uneven luminance as well as uneven chrominance. As well, the differences in refraction of light color can introduce significant divergence in the image colors.